Disturbances in the form of quasi-stationary tones having an harmonic spectrum, such as hum for example, often create problems. An example of one area in which such problems arise is the field of electronic music. Many electro-mechanical musical instruments, inter alia electric guitars, have magnetic pick-ups which readily capture the magnetic hum-field present in the vicinity of mains supply cables and mains transformers. Some musical instruments have poor mains components, which also results in humming tones from loudspeakers. The cables in the loudspeaker systems of auditoriums are also liable to cause hum in the mains network.
Another area in which such problems arise is that of medical technology, in which extremely weak electric signals from the human body are measured, such as in electrocardiograms for example. Disturbances in the form of mains hum also occur when transmitting electric signals over long distances; in systems which incorporate sensitive transducers; and applications which utilize extremely low voltages or current strengths.
Experience has shown that there are many different ways of attempting to eliminate hum disturbances of the aforesaid kind.
One method in this respect is to remove the source of disturbance, or to rotate the source so as to minimize the hum transmission coupling.
The electrical system may be screened and/or earthed more effectively.
When the signals are transmitted through cables, balanced cables may be used.
Hum which is caused by poor rectifiers in the instrument can be eliminated by re-designing the mains components of the instrument.
The strongest harmonic components of the hum can be cutoff with the aid of high-pass filters
Despite these many methods, however, it can still be difficult to eliminate hum to a satisfactory extent. For example, in the case of a guitarist who constantly moves around the stage, it is practically impossible to eliminate hum transmission effectively. In the case of loudspeakers in auditoriums, the cost of changing the wiring system can be very high. Balanced cables will only reduce hum when used together with balanced signal sources and balanced receiver inputs, which impairs flexibility. The cost of re-designing the mains components of an instrument can also be extremely high. In the case of high-pass filtration, the useful signal is also affected and when the hum is rich in harmonics, high order harmonic components are not eliminated.
The object of the present invention is to provide an arrangement with which these and other drawbacks can be overcome.